A list of prior patents and publications which may be of interest is provided below:
______________________________________ U.S. Patents Pat. No. Inventor Issue Date ______________________________________ 3,500,733 Kreuz et al 03/1970 3,950,097 Gopfert et al 04/1976 4,020,344 Kerschbaum 04/1977 4,143,400 Heckman, Jr. et al 03/1979 4,199,869 Van Steenwyk 04/1980 4,313,678 Colvocoresses 02/1982 4,922,254 Schuessler et al 05/1990 5,019,828 Schoolmar 05/1991 5,101,268 Ohba 03/1992 5,140,532 Beckwith et al 08/1992 5,172,233 Yoshihara et al 12/1992 5,251,037 Busenberg 10/1993 ______________________________________